There are a large number of situations where it is desirable to be able to measure small variations (e.g. of the order of .+-.0.5 microns) in the dimensions of surfaces.
One such situation is the measurement of the diameter of core samples from a subterranean rock formation. As explained in U.K. Patent Application No. 8911773, stresses in subterranean rock formations may be determined by measuring residual strains in core samples recovered from the formations. If the recovered core is cut exposing a fresh surface and relieving the stress, the core will expand with the greatest expansion occuring in the areas where it was subjected to the greatest stress and the least expansion occurring in the areas of least stress. In this U.K. Application the measurements are made using strain gauges placed on a face of the core. Measurement using strain gauges is an accurate but time consuming method. Stress determination using this basic method would be simplified and speeded up if an instrument capable of direct measurement of the expansion over different parts of the core was available.
There are a number of potential ways of measuring small changes in the dimensions of surfaces and objects and some instruments are available on the market. All the current methods and instruments have potential drawbacks, however, as summarized below: